In the, conventional process for fabricating seamless tubing with multi-layered metals, coextrusion is generally applied. The extrusion billet is assembled with multiple layers of metallic cylinders one inside the other concentrically. The annular opening at each end of the assembly is sealed by vacuum welding. The billet assembly is preheated to a proper temperature and extruded, forming a metallurgical bond between adjacent layers. The bondline defects are occasionally detected at the as-extruded stage or after the tube is further reduced. The main cause for this problem is due to insufficient time at temperature to effect bonding and diffusion across the bondline during extrusion.
In the conventional process for preparation of two-layered zirconium-zirconium alloy billet, the outer zirconium alloy cylindrical part is first subjected to beta heat treatment by heating at 980 degrees C. to 1200 degrees C. range and water quenching in order to improve corrosion resistance and to achieve fine grain structure for the final tube product. After the billet is assembled with an inner zirconium liner, it is extruded to a tubeshell and further reduced by tube reducers to final size suitable for nuclear fuel cladding applications. A stringent inspection such as ultrasonic technique is used for bondline defects which may be as small as a few thousands of an inch in dimension.
It is, therefore, an objective of the present invention to reduce or eliminate the occurrence of bondline defects in concentric mating surface for tubing with multiple metallic layers.
It is a further objective of the present invention to promote maximum bond strength with diffusion across the interface between the inner and outer metallic layers.